Small articles such as dolls, balls, shoes, and the like have been molded from liquid polymer systems including rubber and vinyl resins. For example, U.S. Pat. No. 1,998,897, granted in 1935 discloses a "rotational casting" method of manufacturing hollow articles from a heat-sensitive latex. In this method, a measured charge of the heat sensitive latex is deposited in a closed mold which is then rotated in a plurality of planes and as the mold rotates, the latex flows and coats the inner surfaces of the mold. Simultaneously with the rotation of the mold heat generally from 300.degree. F. to 400.degree. F. and even higher is applied in order to get the latex in the form of a thin skin on the inner walls of the mold. The gelled product is then removed from the mold.
Later approaches suggested in Modern Plastics, Volumn 26, page 28, of April, 1949, discuss rotationally casting a plastisol by depositing a measured charge of plastisol into a closed mold and rotating the mold in a plurality of planes. While the plastisol is heated, e.g., up to 400.degree. and even higher using internal means to flux it, it forms a thin skin over the inner wall of the mold. Later the source of heat is removed, the mold is cooled, and the article removed.
Another approach to the manufacture of hollow articles using a plasticized thermoplastic resin is disclosed in U.S. Pat. No. 3,347,371. According to this process, a thermoplastic resin together with a plasticizer and optionally other materials are preblended in a mixer into particles of sufficiently small size so that when introduced into a rotating mold, the material is dry but fluent. A premeasured charge is introduced into the mold and the charged mold is rotated in a plurality of planes and simultaneously heated, e.g., up to 400.degree. F. and even higher, causing the particles contained therein to coalesce and distribute in a relatively even, coalesced layer over the interior surface of the mold in the form of the object to be produced. U.S. Pat. No. 3,420,729 also discloses various compositions suitable for molding as well as heating of a mold while it is being rotated. Similarily, U.S. Pat. No. 3,683,062 discloses heating or cooling of molds at indexed stations as well as rotation in two axes. Various compositions which have been rotationally molded by the prior art methods have been disclosed in U.S. Pat. No. 3,414,456.
Centrifugal casting in which a moving mold rotates about only 1 axis is disclosed in U.S. Pat. Nos. 3,222,439; 3,410,934; and 3,551,551. In these patents the effects of centrifugal force upon the polymer or polymer systems so rotated is a significant factor in producing the result desired and to this end high unidirectional rotational speeds are employed.
In summary, then, the rotational molding of plastics using thermoplastic polymers is already known, wherein a quantity of powdered or liquid thermoplastic material is placed into a closed metal mold and the mold is introduced in an oven maintained at high temperature. While the mold is heated, it is rotated in two dimensions, heating the material and causing the material inside the mold to fuse and coalesce in place. Temperatures of the order of about 450.degree. F. or even higher have been used.
Once the mold walls have been uniformly coated by the heated material, the mold is cooled or the formed article is removed from the oven and allowed to cool. For instance, the mold is set out into the room and allowed to cool to room temperature. When sufficient cooling has occured so that the walls of the object are relatively rigid, the finished part is removed from the mold. The resulting product has relatively thin walls.
It will be readily appreciated that in the process described above, where it is necessary to apply heat to the rotating mold, the rotatable apparatus and molds must withstand the high operational temperatures that are about 450.degree. F. or even higher. This requires equipment of strong, heavy, rigid construction, equipment that requires a substantial investment. Additionally, an adequate period of time is generally required for the mold to cool before the molded object is removed therefrom, thus increasing the time necessary to produce each molded article and tying up expensive equipment.